For mutants in general Taban/ Eden miniatures- the degenerates do work well for muties (
link), some of the Bamaka as well(
link).
Madpuppet Miniatures had some nice mutants, though I don't know if the shop still operates- might be better to contact them first before putting in an order.
Some of the
Dark Age minis might work as well.
Hasslefree have some mutants hiding amongst their
zombies, also some of the
aliens might work as well.
Heresy has some lovely
ghouls, that also could be used as mutants
Fenris have a mutant (Amos) and two zombies (Homer & Barney)